The aluminizing process is well known for improving the oxidation and corrosion resistance of many substrates such as alloys containing chromium, iron, nickel, or cobalt, as the major constituent. In particular, aluminide coatings are known to improve the oxidation and corrosion resistance properties of the nickel-and-cobalt-based superalloys which are used in high-temperature environments, such as gas turbine blades and vanes.
In one typical coating process, the article to be coated is embedded in a powder pack containing powdered aluminum, either as the metal, an alloy, or a compound such as cobalt, a carrier, typically an ammonium or alkali metal halide, and an inert filler such as aluminum oxide. Once embedded, the article is heated to between 1400.degree. F. and 2200.degree. F., depending on the particular coating material, with the thickness of the coating depending on the temperature and the duration of the exposure. The halide acts as a carrier or activator to facilitate the transfer of the aluminum from the powder pack to the exposed surface of the article, where the aluminum is deposited. At the surface of the article, the aluminum and the substrate material interdiffuse to form an aluminide coating, and the halide is freed to transport more aluminum from the powder pack to the article. As the coating thickness increases, the interdiffusion of the aluminum and the substrate decreases, thereby increasing the percent by weight of aluminum in the aluminide coating.
Another coating process, referred to as out-of-pack gas phase deposition of aluminized coatings, is described in U.S. Pat. Nos. 3,486,927 and 4,148,275, which are hereby incorporated by reference herein. This process is similar to the powder pack method except that the article is not embedded in the powder pack. Instead, the aluminum-bearing halide gas is circulated from the powder pack using an inert gas, and into contact with the article to effect an aluminum deposition on the exposed surfaces of the article, producing a coating of substantially uniform thickness thereon.
The out-of-pack process is very useful in applying aluminide coatings to the airfoil section of gas turbine blades. Turbine blades so coated demonstrate significantly greater oxidation and corrosion resistance than uncoated blades, increasing the useful life of the turbine blade. Since the protection from oxidation and corrosion provided by the aluminide coating is directly related to the thickness of that coating, it is desirable to further increase the thickness of the aluminide coating on the airfoil, where that protection is needed most.
However, as the thickness of the aluminide coating increases, the commensurate increase in the percent by weight of aluminum reduces the ductility of the coating. Due to the highly stressed nature of the turbine blade platform region adjacent the pressure side of the airfoil, the aluminide coating in this region becomes susceptible to fracturing during blade use if the coating thickness exceeds a maximum allowable thickness. The nature of this fracturing is such that cracks in the coating readily propagate into the substrate of the blade platform itself, reducing the integrity, and therefore the useful life, of the turbine blade.
Unfortunately, the aluminide coating thickness necessary to provide the desired oxidation and corrosion resistance on the airfoil is significantly greater than the maximum allowable coating thickness in the blade platform region adjacent the pressure side of the airfoil. Since the out-of-pack gas deposition method produces a coating of substantially uniform thickness, the aluminide coating thickness on the airfoil has heretofore been limited by the maximum allowable coating thickness in the blade platform region. Consequently, the oxidation and corrosion resistance of gas turbine blades and vanes of the prior art is significantly less than that which could be obtained if the blade platform coating thickness were not a limiting factor.